1. Field of the Invention
The present invention relates to a bearing unit in which a magnetic fluid is used as a lubricant, and more particularly to a bearing unit suitable for motors for audio equipment, motors for video equipment, polygon motors for laser scanners, fan motors for air-conditioning equipment, spindle motors for magnetic disk drives, and other motors which are required to cause no contamination of the equipment due to leakage of a lubricant and further to satisfy the requirements on less noises, high-speed and high-accuracy rotations.
2. Description of the Prior Art
As a bearing unit for general small size motors there has been known a rolling bearing or a slide bearing using air or an oil as its lubricant. In the fields including audio equipment, video equipment, laser beam printers, magnetic disk units and air-conditioning equipment, in keeping with the improvements of product qualities and multifunctions, their motors are being required to meet the requirements on high-speed and high-accuracy operations. As for the improvement of the high-speed performance, for example, there have been made the approaches to increase the speed of a cylinder motor for a VTR from 1800 rpm up to 10000 rpm, the speed of a polygon mirror motor for laser scanners from several thousands rpm up to several ten-thousands rpm, and the speed of a spindle motor for magnetic disk drives from 3600 rpm up to 10000 rpm. In addition to these requirements, these motors have been exposed to the requirements on high rotational accuracy. However, it is difficult for rolling bearings to accommodate such a high-speed rotation in terms of the rotational accuracy. For this reason, slide bearings of fluid lubrication type has come into practical use.
On the other hand, such equipment can not accept oil leakage from the bearing, so that it strictly requires a great ability of the oil seal. The slide bearing using an oil lubrication can be expected to exhibit a high rotational accuracy because of having an excellent bearing rigidity and an excellent dumping property, though contamination is unavoidable due to oil leakage and oil mist. To solve this contamination problem, there has been proposed a magnetic fluid bearing in which a magnetic fluid is employed for its lubrication and seal. Japanese Unexamined Patent Publication No. 59-126114 proposes a magnetic fluid bearing constructed such that a magnetic fluid is impregnated into an oil sintered bearing of a porous material and held on a sliding surface by means of a permanent magnet to maintain the fluid lubrication as well as to provide a sealing function.
A magnetic fluid bearing, in which a magnetic circuit uses a permanent magnet to hold a magnetic fluid, is such that the magnetic fluid impregnated expands due to temperature rise of the bearing caused by its high-speed revolution and hence exudes, which increases a centrifugal force applied to the magnetic fluid exuded from the bearing. For this reason, there is a possibility that the magnetic fluid may scatter from the bearing section and the seal portion. In addition, temperature rise of the bearing section causes the magnetic fluid and air within the bearing unit and others to expand to raise the pressure within the bearing unit. This requires the use of sealing structure capable of standing this pressure rise, and ordinary magnetic fluid seals can result in the lubricating oil exuding and scattering from the sealing section at a high-speed rotation. Moreover, ordinary porous oil sintered slide bearings fail to supply a sufficient amount of lubricant onto sliding surfaces, thus making it difficult to keep high-accuracy rotations. Accordingly, to apply the magnetic fluid bearing unit to the above-mentioned motors, there is a necessity to provide a sealing structure and an oil supply mechanism which can accommodate high-speed rotations.